ULF Turbulence in the Neptunian Polar Cusp

W. M. Farrell, R. P. Lepping and C. W. Smith

Journal of Geophysical Research, A98, 3631-3643 (1993)

Abstract:

One of the most fortuitous events occurring during the Voyager 2 Neptune encounter was the passage of the spacecraft through the southern magnetic cusp region prior to entry into the magnetosphere. This region was previously identified by a slow, steady change in the magnetic field direction to a dipolar configuration and a slow, steady decrease in electron and ion fluxes. Such observations are similar to those of the terrestrial cusp. Besides these characteristics, the region also possesses enhanced ULF magnetic turbulence at frequencies less than 0.5 Hz as measured by the Voyager 2 magnetometer (MAG) experiment. The activity is particularly strong at the frontside magnetopause boundary and at the backside cusp/magnetosphere boundary. Specifically, in these regions the fluctuation level, (delta B)/B, is as large as 6%. The enhanced ULF turbulence extends from 0.01 Hz continuously up to nearly 0.5 Hz, and is consistent with the whistler mode. Calculations indicate that such turbulence should resonate with electrons having energies in the tens of kiloelectron volts. Observations indicate a very strong correlation of the ULF turbulence with the energetic electrons between 22 and 35 keV measured by Voyager's low-energy charged particle (LECP) experiment. This result suggests a vigorous interaction between the two. Based on an assumed cusp geometry and measured wave levels, the ULF turbulence could generate as much as 10(exp 7) W of power that would be absorbed by the surrounding magnetosphere. This power input represents about 1% of the total auroral power required to drive the UV aurora and radio emissions. Thus the ULF wave turbulence in the cusp may represent a significant but not complete power source for the magnetosphere.